Examples of such currently-used apparatus include an inkjet printing apparatus having a transport path transporting web paper, a print unit disposed along the transport path and discharging ink droplets to the web paper for image formation, and an image processor outputting print data to the print unit. See, for example, Japanese Unexamined Patent Publication No. 2013-176868A. The print unit includes three printing heads individually spaced away from each other in a transportation direction of the web paper.
The inkjet printing apparatus with such a configuration may cause the web paper to skew upon transportation of the web paper on the transport path. When printing is performed under this state, a positional shift (also referred to as a misregister) may occur in image data between an intended positional relationship and a positional relationship of ink droplets discharged from the upstream printing head and those discharged from the downstream printing head. Accordingly, the image processor performs correction by shifting the image data in a width direction of the web paper in accordance with a skew amount of the web paper, and outputs correction image data to the print unit. Then the print unit performs printing in accordance with the correction image data.
However, the example of the currently-used apparatus with such a configuration possesses the following problem.
That is, the currently-used apparatus shifts the web paper per nozzle of the print unit in accordance with the skew amount. Consequently, the correction is sometimes performed unsatisfactorily in accordance with a skew amount having a value of a non-integral multiple of a nozzle interval, which may cause insufficient suppression of the positional shift. Here, the insufficient suppression of the positional shift leads to reduction in printing quality. As a result, the suppression of the positional shift is an important problem.